Platinum is considered the best electrocatalyst for the four-electron reduction of oxygen to water in acidic environments because it provides the lowest overpotentials and the highest catalyst stability. However, even on pure Pt, potentials in excess of 300 mV are lost from the thermodynamic potential for oxygen reduction due to competing water activation and sluggish oxygen reduction kinetics. Furthermore, oxygen undergoes non-dissociative adsorption on Pt metals accompanied by some dissociative adsorption, which results in Pt oxidation. Importantly, Pt remains an expensive metal of low abundance.
There has been considerable research on non-precious metal catalysts such as: (i) porphyrin-based macrocyclic compounds of transition metals (e.g., cobalt phthalocyanines and iron tetramethoxyphenyl porphyrin (Fe-TMPP)), (ii) vacuum-deposited cobalt and iron compounds (e.g., Co—C—N and Fe—C—N), and (iii) metal carbides, nitrides and oxides (e.g., FeCx, TaOxNy, MnOx/C). Pyrolysis at higher temperatures than 800° C. in an inert or NH3 atmosphere led to the improvement in catalytic activity of the catalysts to some extent, but none of the above catalysts are active enough to be used for oxygen reduction catalysts in low temperature fuel cells such as polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs).
There is disagreement in the literature regarding the catalytic reaction site for oxygen reduction and the relevant mechanism on non-precious metal catalysts. The most commonly accepted hypothesis is that the metal-N4 center bound to the carbon support is catalytically active, and the central metal ion in the macrocycle plays a crucial role in the oxygen reduction reaction. It has been proposed that oxygen reduction on N4-chelates of transition metal occurs via a modified “redox catalysis” mechanism. That is, an oxygen molecule is adsorbed on the catalyst metal center to form an oxygen-catalyst adduct, followed by electron transfer from the metal center and the regeneration of the reduced N4-chelates. From the analysis of Fe-based catalysts by Time-of-Flight Secondary Ion Mass Spectrometry, Dodelet and his coworkers [J. Electrochem. Soc., 153, A689 (2006)] have maintained that two different catalytic sites, i.e., FeN4/C and FeN2/C, coexist in the catalysts, irrespective of the Fe precursors used. Here, FeN4/C represents an Fe ion coordinated to four nitrogen atoms of the pyrrolic type, and FeN2/C stands for an Fe ion coordinated to two nitrogen atoms of the pyridinic type.
On the other hand, Yeager [Electrochim. Acta, 29, 1527 (1984)] and Wiesener [Electrochim. Acta, 31, 1073 (1986)] have suggested that the transition metals do not act as an active reaction site for oxygen reduction, but rather serve primarily to facilitate the stable incorporation of nitrogen into the graphitic carbon during high-temperature pyrolysis of metal-nitrogen complexes. This means that high-temperature pyrolysis in the presence of transition metals yields a carbonaceous layer with substantial nitrogen groups that are catalytically active for oxygen reduction.
Nitrogen-containing carbons have been typically prepared using implantation through NH3 or HCN treatment of carbon at high temperatures. Another way to prepare carbons with a controlled nitrogen content is to synthesize carbon powder using nitrogen-containing polymer precursors, followed by physical or chemical activation process. The experimental measurements of nitrided Ketjen black indicated an onset potential for oxygen reduction of approximately 0.5 V(NHE) compared to that of 0.2 V(NHE) for untreated carbon. It is widely believed that the two electron transfer pathway is dominant on most of nitrogen-doped carbons, producing a large amount of H2O2, since the O—O bond breakage is not feasible.
As such, a need currently exists for a low-cost, easily manufactured carbon-based catalyst having high activity, selectivity, and stability for oxygen reduction.